1. Field of the Invention
The present invention relates to a method for producing a high-quality silicon wafer in which, in a region outside of a device active region, capable of performing gettering of metallic impurities (contaminants) such as impurities originated from raw materials of a silicon single crystal and dissolved in the silicon single crystal and impurities mixed in the silicon wafer during a device production process, and relates to a method for producing a high-quality semiconductor device which does not easily suffer from an influence of impurities in the silicon wafer. Priority is claimed on Japanese Patent Application No. 2006-038485, filed Feb. 15, 2006, the content of which is incorporated herein by reference.
2. Description of Related Art
Silicon wafers have been widely used as substrates of semiconductor devices constituting electronic devices such as portable telephones and portable communication devices. Conventionally, widely used silicon wafers had a thickness of 100 μm or more, especially 700 μm or more. However, the demand for thinning the thickness of a silicon wafer has increased in recent years in accordance with the recent development of high-performance, multifunctional electronic devices. For example, a current model of portable telephone is generally equipped with a so-called “electric eye” such as a CCD or a CMOS image sensor and a memory for storing image data obtained by the CCD or the CMOS image sensor. In order to realize such a constitution, silicon wafers used in a transceiver unit, a CCD substrate, a memory substrate or the like are worked to thin films of not thicker than 100 μm (in 2006, a thickness of less than 50 μm was realized), and stacked to form a multi-layered (plural layered) structure and are packaged. The techniques including film-thinning, multi-layer stacking, and packaging are generically called SIP (system in package) or MCP (multi chip package).
The film-thinning of a silicon wafer has been realized by grinding (back-grinding) of a back surface of the wafer using CMP or the like after the device formation process for forming a semiconductor device on a front surface of the silicon wafer. Where the thickness of the silicon wafer is controlled to be 100 μm or less by grinding, residual strain accompanied with residual stress has a substantial influence on the mechanical strength of the silicon wafer at the time of bonding of the silicon wafer. Therefore, the residual strain is removed after grinding the silicon wafer.
In addition, it is well known that contamination of a silicon wafer with metallic impurities (contaminants) deteriorates the electric properties of a semiconductor device. Therefore, there have been proposed various techniques to capture the contaminants in the silicon wafer in a region outside of a device active region. While growing a silicon single crystal, metallic elements dissolved from the raw materials contaminate the silicon single crystal. Such impurities constitute contaminants of a silicon wafer sliced from the silicon single crystal. In addition, metallic impurities such as Cu or the like contaminate the silicon wafer while forming a wiring on the silicon wafer.
As a technique for gettering of metallic impurities (contaminants) in a silicon wafer, for example, there is a technique for forming oxide precipitates by precipitating oxygen atoms contained in a silicon single crystal grown by the Czochralski method (CZ method) and utilizing strains around the oxide precipitates as the gettering sink, and thereby capturing the metallic impurities (contaminants).
In another technique, strains in polycrystalline grain boundaries of a polycrystalline film formed on the back surface of the silicon wafers are used as the gettering sink for capturing metallic impurities (contaminants).
In another technique, a film of a metal having a high melting point is provided on a back surface of a silicon single crystal wafer so as to provide an ability for gettering of impurities in the silicon single crystal (for example, Patent Reference 1: Japanese Unexamined Patent Application, First Publication, No. 2002-313795).
In addition, another technique is proposed for a method for gettering of impurities composed of transition metals that diffuse very rapidly in a silicon wafer and form a deep impurity level, especially for gettering of Co, Ni or Cu, which diffuse very rapidly at room temperature. In this technique, the silicon is doped with two types of impurities, oxygen (O) and carbon (C), and is subjected to thermal annealing. As a result, impurity composites consisting of carbon, oxygen, and transition metals are formed in specific atomic sites in the silicon crystal (For example, Patent Reference 2: Japanese Unexamined Patent Application, First Publication, No. 2003-209114).
However, since the above-described prior arts require a heat treatment for gettering, there have been problems of high production cost and labor accompanied with the heat treatment.
Where the oxide precipitate (BMD: Bulk Micro Defect) is used as a gettering sink, it is necessary to perform a heat treatment so as to grow BMDs. While the heat treatment for growing BMDs may be performed before the device process (a process for producing a device), there is a problem of production cost and labor accompanied with the heat treatment. Alternatively, for gettering of metallic impurities, the BMDs may be grown by a heat treatment in the device process. However, recently, there is a tendency to perform the device process under low temperature conditions with the intention of cost reduction. In such a case, without performing an additional heat treatment for gettering, there is a possibility that a sufficient heat treatment margin for gettering of metallic impurities (contaminants) cannot be obtained in the device process.
Where thinning of a silicon wafer is performed by grinding, the residual strain caused by the grinding and accompanied with the residual stress acts as a gettering sink of metallic impurities generated during the device process. If the residual strain accompanied with the residual stress is removed, the gettering sink is also removed and the contaminants released from the gettering sink diffuse into the device active region and deteriorate the electronic properties of the semiconductor device. A solution to this problem is to perform the heat treatment after the film-thinning of the wafer so as to grow the BMDs and perform gettering of the metallic impurities (contaminants) by the BMDs. However, it is highly possible that the heat treatment after the film-thinning causes warpage or cracking of the wafer.
Further, where Cu constitutes a contamination source, since Cu+ migrates in a silicon wafer even at room temperature, there is a possibility that desired product properties cannot be obtained because of the migration of contaminants after the device process, for example, in a process for forming a semiconductor chip, or in a process of stacking of chips to form, e.g., flash memories.
Based on the consideration of the above-described circumstances, an object of the present invention is to provide a method for producing a silicon wafer which exhibits an effective gettering of metallic impurities (contaminants) even after the silicon wafer is made into a thin film by grinding of the wafer, does not require heat treatment for gettering, and does not cause a deterioration of product properties.
Another object of the present invention is to provide a method for producing a semiconductor device which does not easily suffer from an influence of contaminants in the silicon wafer and has high quality.